They fear that nerve agents and chemical weapons held by forces loyal to President Bashar al-Assad’s regime could fall into terrorists’ hands if the government collapses entirely.

Senior officers have also held talks on a range of “rogue state” contingency plans to prevent chemical, biological or nuclear weapons from being seized by terrorists, which they fear could also happen if Pakistan or North Korea’s regimes were to collapse.

Iran, which according to one senior British source is “bent on developing nuclear weapons”, is also causing great concern to western governments.

British intelligence believes Syria has amassed an extensive arsenal of WMD including nerve agents such as Sarin – one of the most deadly weapons ever created – and chemical weapons such as mustard gas.

They have so far not been used and are currently considered to be well guarded by the Syrian security forces.

However militant Islamist groups are already inside Syria fighting against the government and would be perfectly placed to raid WMD stockpiles, according to intelligence sources.

Sources have said that the most likely option to prevent WMD falling into the hands of extremists would be to destroy stockpiles in a series of air strikes.

Alternative options include the use of special forces and troops trained in chemical warfare to secure WMD sites in Syria if and when the government eventually collapses.

An RAF Regiment unit called the Defence Chemical Biological Radiological and Nuclear Wing based at Winterbourne Gunner, Wilts, has already been warned that it should be prepared to work alongside the SAS in securing WMD sites in the Syria at short notice.

Last week a US-based body known as the Strategic Working Group began rehearsing how WMD stockpiles would be secured in both the Middle East and the Pacific in the event of an international emergency.

The group is composed of military personnel from the US Army, Marine Corps, Navy as well as British and Australian officers and government officials.

The senior officers tested a variety of plans at a classified war gaming session called Unified Quest 2013 at the US Army Staff College at Fort Leavenworth in Kansas.

The scenario focused on a failed state that has lost control of its WMD stockpiles, forcing the United States and other countries to intervene.

The location of the game was classified, but informed opinion suggested that North Korea was the target country.

One source who took part in the war games said: “We need to have plans in place so that we can properly prepare our soldiers for this job. It’s a dangerous and messy business.

"Soldiers will be driving into potentially contaminated areas, possibly under fire while handling hazardous material.”

MI5, Britain’s security service has repeatedly warned that it is “only a matter of time” before extremist groups carry out a “chemical, biological or radiological attack” on a western city.

Such an attack was also identified as a “Tier Two Priority Risk” in the 2010 National Security Strategy.

Defence sources said that one of the unintended consequences of the Arab Spring was the huge volume of illicit weapons which have entered the illegal arms market, increasing concerns about what could happen if Assad lost control of his WMD.

A source said: “After Libya collapsed thousands of man portable air defence weapons went missing and these can bring down an airliner. [there were 5,000 sorites by NATO planes to ensure Qaddafy didn't use his airforce -- why did they not take the occasion to destroy as much of Libya's weaponry in warehouses and at airports as they could?]

"We know Syria has a pretty extensive armoury and a lot of chemical weapons. We need to ensure these do not enter the terrorist food chain.”

Both British and US commanders agree that the West has paid “lip service” to training troops in WMD scenarios and has focused almost solely on counter-insurgency operations such as those undertaken in Afghanistan and Iraq.

One senior British source added: “Syria has a sizeable arsenal of chemical weapons including nerve agents and mustard gas.

"Pakistan and North Korea have nuclear weapons and it is widely believed that Iran also intends to develop a nuclear weapon.

“These are all unstable or unpredictable states and the potential for WMD ended up in the hands of terrorists is very real. We need contingency plans to deal with a wide variety of scenarios.”